The figure on the booking form covers the tradesman reaching you and the first hour of work, and it is paid at the end of the visit against a numbered docket.
If the job needs quoting, the quotation is written on the spot and the call charge you have already paid is deducted from it. Accept the quotation and you have paid for that hour once. Decline it and you keep the written figure and owe nothing further.
We were doing it the other way round for a year and it made every visit feel like a sales call. This arrangement means the tradesman has no reason at all to find you more work than you have.